Up to now, molten salts are often used for heat transport and heat storage at higher temperatures. As a drawback, they are very aggressive at temperatures above about 550 °C and lead to heavy corrosion. However, higher temperatures are necessary to improve the efficiency of solar-thermal devices. In contrast, sodium is a good candidate for high temperature applications. Its physical material properties are well suited; among others, its melting point is at about 100 °C and its boiling point at nearly 900 °C at atmospheric pressure, giving a wide temperature range for single-phase applications without the need to design high-pressure systems. In addition, it is more appropriate for applications with cyclic transients. It is a proven coolant technology on industrial scale, and our institute worked to this aspect for many years. As a disadvantage of sodium, the interactions with water and with air in case of leakages have to be considered.